where \(E\)  is the electron energy. This parameter represents the electric field strength \(\mathcal {E}\)  in units of the critical Schwinger field in the co-moving frame of the electron. For an electron \(\mathcal {E}_S = 1.3\times 10^{18}\)  V/m. This parameter is a product of the two previous ones Eq. (REF ) and Eq. (REF ). It has been shown that quantum effects become non-negligible already for \(\chi \gtrsim 0.1\)  [1]}.
